1. Field of the Invention
This invention relates to a self-vibrating dc-operated vibrator motor used for a pocketable wireless alerting or communication device such as a pager or a portable phone. The motor has an eccentric armature rotor for creating vibration.
2. Description of the Prior Art
FIG. 10 shows a typical prior known dc vibrator motor. In FIG. 10, a cylindrical coreless dc vibrator motor MG has an output shaft S6 on which an eccentric weight W, made of a tungsten alloy etc., is fixedly mounted. The motor vibrates as the shaft S6 rotates because of the unbalanced distribution of the mass about the rotational axis.
However, some danger of mechanical interference pertains to this type of vibrator motor since the rotating eccentric weight is located outside the motor case. In addition, the alerting device requires an extra space in which the eccentric weight turns, and the high density material, such as tungsten alloy, for the eccentric weight is costly.
As disclosed in U.S. Pat. No. 5,036,239 (issued Jul. 30, 1991, one of the present applicants invented a flat coreless dc vibrator motor having neither output shaft nor external weight. This motor has a rotor which incorporates generally sector shaped non-overlapping flat armature coils arranged angularly partially about the shaft of the rotor, so that the center of mass of the rotor is off the rotational axis of the rotor.
As disclosed in U.S. Pat. No. 5,107,155 (issued Apr. 21, 1992), the same applicant invented a dc vibrator motor having neither output shaft nor external weight. This motor has a fixed shaft on which a cylindrical magnet is fixedly mounted, and an eccentrically-weighted rotor having a cylindrical coreless windings, enclosing the fixed magnet, is mounted on the fixed shaft. The rotor has either eccentric bearings or a combination of eccentric bearings and eccentric weights, so that the center of mass of the rotor is off its rotational axis. Although the motor has been favorably accepted in the market, the rotor of this motor requires additional production steps.
As disclosed in U.S. Pat. No. 5,341,057 (issued Aug. 23, 1994), two out of the present applicants jointly invented an additional do vibrator motor having neither output shaft nor external weight. This motor has an eccentric armature rotor having three-pole armature iron cores and coils that are all situated within an angular range of 180.degree. about its rotational axis.
A sector-shaped eccentric armature rotor having all of the armature poles within an angular range of 180.degree. provides a good mass unbalance. However, the iron armature cores of the motor disclosed in U.S. Pat. No. 5,341,057 causes a cogging torque and requires comparatively high voltage for starting.